Memory cells are difficult to functionally define in human subjects. Humanized mice that have received implants of human thymus and fetal liver generate a human immune system and spontaneously respond to endogenous and environmental antigens to generate cells with the phenotype of presumed IgM memory/activated B cells and class-switched memory/activated B cells. We will use an immunization and BrdU labeling strategy to identify antigen-specific B cells that retain BrdU label over extended periods in order to functionally define human IgM memory and class-switched memory cells. Controversies regarding the origins of human IgM+CD27+ B cells may also be resolved in similar studies on humanized mice. PUBLIC HEALTH RELEVANCE: Relevance Defining and understanding the origins of human memory B cells will help the development of novel and rational approaches for vaccine generation. These cells are also linked to a number of human B lineage malignancies and understanding the biology of these naturally long-lived B cells will be of importance from a lymphomagenesis standpoint.